The present invention relates to a novel method of photohardening a composition, to novel photohardenable compositions, to photosensitive materials employing them and to methods for two and three dimensional imaging using the same. More particularly, it relates to a chemical method using two photons to harden compositions containing certain photochromic materials by free radical addition polymerization.
The invention further relates to dosimetric devices incorporating benzospiropyrans.
Though it is relatively simple to transmit two dimensional drawings, figures and photographs from one place to another by optical processes such as xerography and photography, three dimensional photography or xerography is essentially unknown. Prior to the present invention three dimensional figures have been made visible or real by a number of different methods. Prior to the present invention, the techniques for doing so have been slow and expensive.
U.S. Pat. No. 4,575,330 to Hull and assigned to 3 D Systems describes a method for forming a three dimensional objec called stereolithography. Stereolithography involves the formation of a three dimensional object additively, i.e., layer by layer. As such, it couples the power of computer graphics, through laser initiated photopolymerization to the formation of a unique, real plastic form Stereolithography, in its original manifestation, employs a He/Cd laser (324 nm), a UV photoinitiator (usually an acetophenone acetal) and a mixture of viscous polyolacrylates. The x,y dimension is controlled by scanning the laser beam over the surface of the polyolacrylate/UV initiator mixture at a rate sufficiently fast to form a gelled layer of fixed depth on an aluminum platform immersed to essentially 100 .mu. in the polyolacrylate/UV initiator mixture After formation of the first layer the platform is immersed to a depth of 200 .mu. and a second layer is formed on top of the first. This process is continued in an additive modeling fashion until the unique object is formed. The entire x,y pattern for each of the successive layers of the model is controlled from a computer --either by vector scanning or raster scanning the laser beam from point X.sub.1,Y.sub.1 to X.sub.n,Y.sub.n.
Among the applications of stereolithography is the formation of models from CAD/CAM. One of the problems in the aforesaid process is that all of the photochemical polymerization events are required to take place at the interface between the monomer and the air. The mixture cannot be penetrated by the initiating beam without causing a polymerization process. In all cases of application to date, the photopolymerization events are carried out on a platform located a small distance under the surface of the photohardenable mixture. At the completion of the formation of a single layer, the platform is dipped further under the surface to expose the top of the formed layer of polymer to additional monomer mixture. This causes a dislocation in the level of the surface layer which comes to equilibrium slowly if left alone, or which can be leveled with a wiping blade artificially. The overall slow step in the three dimensional modeling process when viscous polyolacrylates are used as the prepolymer is the releveling process. It takes a substantial time period for the polyolacrylate mixture to come to dimensional equilibrium after the movement of the platform into the vessel following the formation of the first layer of stereolithogram. It would be desirable to extend the current practice of stereolithography to the formation of polymer internally of a volume of polymerizable composition rather than on its surface and thus obviate the time consuming reequilibration of the surface of a viscous monomer.
U.S. Pat. Nos. 4,041,476, 4,078,229, 4,238,840, 4,466,080, 4,471,470, an 4,333,165 to Swainson and assigned to Formigraphics Engine Corporation disclose a number of concepts which relate to the current invention in which a three-dimensional figure is formed in situ in a medium by causing two dissimilar radiation beams to intersect in the media. In this process, the X, Y and Z axes are scanned within a volume of a photohardenable material. There is no stage and scanning is not limited to the X, Y plane as it is in the Hull process. While the Swainson concepts are interesting in theory, the photochemistries disclosed in the patent do not appear to be commercially satisfactory at least from the standpoint of three dimensional model formation.